This disclosure relates to methods and compositions for servicing a wellbore. More specifically, it relates to methods and compositions for use in a wellbore penetrating a subterranean formation.
Natural resources such as gas, oil, and water residing in a subterranean formation or zone are often recovered by drilling a wellbore into the subterranean formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe (e.g., casing) may be run in the wellbore. The drilling fluid may then be circulated through the interior of the pipe and out of the subterranean formation through the annulus formed between the subterranean formation and the pipe. Next, primary cementing may be performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed.
Degradable polymers are increasingly becoming of interest for use in various subterranean applications based, at least in part, on their ability to degrade and leave voids, act as a temporary restriction to the flow of a fluid, or produce desirable degradation products (e.g., acids). One particular degradable polymer that has received recent attention is poly(lactic acid) (“PLA”) because it is a material that will degrade downhole in aqueous media after it has performed a desired function or because its degradation products may be used to perform a desired function (e.g., degrade an acid soluble component, or lower fluid pH to breakdown borate crosslinked fluids).
Degradable polymers may be used to leave voids behind upon degradation to improve or restore the permeability of a given structure. For instance, a proppant pack may be created that comprises proppant particulates and degradable polymers so that, when the degradable polymer degrades, voids are formed in the proppant pack. Similarly, voids also may be created in a set cement in a subterranean environment. Moreover, degradable polymers may be used as a coating to temporarily protect a coated object or chemical from exposure to the subterranean environment. For example, a degrading agent or some other treatment chemical may be coated, encapsulated, or encaged with a degradable polymer and used in a subterranean operation such that the degrading agent may not be substantially exposed to the subterranean environment until the degradable polymer coating the degrading agent substantially degrades. Another use for degradable polymers in subterranean operations may involve creating downhole tools or parts of downhole tools using solid masses of one or more degradable polymers. In such operations, the degradable polymer may be designed such that it does not substantially degrade until the tool has completed its desired function. In some operations, a tool function may be temporarily delayed by coating with a degradable polymer. Still other uses for degradable polymers in subterranean operations may include their use as diverting agents, bridging agents, and fluid loss control agents.
Generally, degradation of a water-degradable polymer with suitable chemical composition and physical properties, for example PLA, may be most desirably achieved over a time period ranging from about few days to about a few weeks at bottom hole temperatures (“BHT”) of above about 60° C. (140° F.). Unfortunately, many well bores have a BHT that may be lower than 60° C. In these lower temperature environments, a relatively longer time (e.g., weeks or even months) may be necessary for the degradable polymer to hydrolyze and breakdown, which may be undesirable. In other situations, degradable polymers that are stable for desired durations at high temperatures under downhole conditions may be needed. Such materials may be required to be more resistant to hydrolytic degradation (i.e., polymer chain scission due to reactions with water). In such cases, methods to accelerate the reactions with water to breakdown the degradable polymer at the end of an operation in a controlled and predictable manner may be of use. In general, irrespective of the BHT, it is desirable to be able to control and/or design a fluid composition with pre-specified rates and durations for degradation and removal of the degradable polymers employed to accomplish timed events or functions in order to minimize waiting-on-degradation time. It is understood that in order to flowback out or remove the degradable polymer from the location of its placement, it may not be necessary to break it totally down to the monomer level. For effective removal of the degradable polymer at the end of an intended operation, the percentage of polymer degradation needed may be as low as 20%. If used as a plug or filter cake, the degradable polymer may degrade to an extent sufficient to loosen packed particle density so that a flowing fluid may break up and flow out remaining un-degraded particulate material.
The quantities of the degradable polymer required to accomplish a desired objective may depend on, among other things, the type of application. For example, in a diversion operation during multistage fracturing, the amounts of degradable polymer needed may be as high as about 250 to about 500 lbs/1000 gal (“gal/Mgal”). It may be beneficial to reduce the amount of polymer utilized to accomplish a particular operation without sacrificing the intended performance objectives, which may reduce the cost of the.